He learned to fight out of self-defence.
Not that he was in any way reluctant to learn the art of combat, or at least how to hit things and make them stay sliced. But, money what it was in the Gromgard estate, his actual tuition was sparse, his irregular practice sessions solitary. Sometimes one of the adults would ask how he was getting on with it. That was all there was to it.
That was, until his father had a bright idea at the dinner table that Gerda should give him some help. “Show your little brother a few tricks, eh?” To which she grudgingly had to agree.
In theory, this was fine. In practice, Gerda wasn’t a teacher, she was a rockslide with an axe. She was infinitely resentful of having to make time to waste on the ‘brat’ - and he got to feel it. He’d been taught with dummies; Gerda trained with dwarves.
The first time, she immediately knocked him senseless and strolled away laughing. The second time, he knew what was coming. He held out for three minutes. And that was it, for a while, and he had time to nurse his bruises before their father brought it up again in his well-meaning way. If he got better in between, it didn’t take the edge off her contempt (or her axe). His mistakes just incited her more, and she made sure to get it over with faster.
He read up while his bruises and his ego healed. He put in as much out of sight practice as he could and talked* one of the last few guards into helping him. If Gerda happened to be away hunting in the Golden Hills, he could sneak off with whatever weapons she’d left behind. (And if he wasn’t careful doing that, he’d get some impromptu practice as well.) By the time he was able to stay on his feet until she got bored, it wasn’t about survival.
He learned to fight out of self-defence. He learned to fight well for revenge.
(*He made the suggestion, and then stared. The guard was bored and not paid nearly enough to stay at his post in the face of adolescents with glowing red eyes.)